DC-DC converters are generally used to provide a highly efficient regulation of a DC supply voltage. The efficiency of a DC-DC converter drops when the supply-voltage is reduced. There is a point where a linear voltage regulator becomes more efficient than a DC-DC converter. Thus, in order to have optimal efficiency across an entire wide supply voltage range, for example 1.43-3.63 Volts, both a DC-DC converter and a linear regulator may be placed side by side. But both require a large area of silicon real estate and a majority of their areas are taken up by the power pass-transistors. This makes providing both in an integrated circuit device very expensive. Alternatively, a solution to this problem could be in using one or another with either higher supply voltage being in the sweet spot for best efficiency using a DC-DC converter, or a low supply voltage being in the sweet spot for best efficiency using a linear regulator. Some integrated circuit devices with DC-DC converters do employ a small keep-alive linear regulator; but again as the name indicates it is only a keep-alive regulated voltage source, not a fully-function linear regulator.
Generally, DC-DC converters, e.g., switched mode buck converters, and linear regulators are kept separate and completely independent as two different voltage regulator circuits. Since the power circuits of these types of voltage regulators carry a large current, they are usually very big in terms of silicon real estate area, therefore internal solutions may, if at all, offer only a very weak linear regulator to save integrated circuit silicon area.